This constraint can be lifted: https://github.com/haskell/fgl/blob/163b35ab67cf00f807295f1c5740b27eefba166c/fgl.cabal#L94 Build and tests succeed with QuickCheck-2.15.0.1 according to my testing.
This constraint can be lifted:
fgl/fgl.cabal
Line 94 in 163b35a
Build and tests succeed with QuickCheck-2.15.0.1 according to my testing.